With the constantly growing demand for high data rate and the scarcity of radio spectrum resources, frequency reuse technique is more and more adopted in cellular systems. However, high frequency reuse factor inevitably increases the inter-cell interference (ICI) and thus impacts spectrum efficiency.
To solve this problem, many solutions have been proposed in the field, wherein multi-BS MIMO is the most promising solution. Compared with other alternatives such as micro-diversity and fractional frequency reuse, multi-BS MIMO has the advantage of increasing both cell-edge performance and system throughput. Multi-BS MIMO has become an important element in many standards such as IEEE802.16m and 3GPP LTE-Advanced (3rd Generation Partnership Project Long Term Evolution-Advanced).
In a conventional single-BS MIMO system, a base station uses its multiple transmit antenna to send multiple data streams to mobile terminals within a cell. In such system, each base station independently selects mobile terminals. In other words, which mobile terminals a base station selects is unknown to other base stations.
In a multi-BS MIMO system, joint selection for mobile terminals among coordinating base stations is required to optimize performance. However, the selection strategies for mobile terminals in the single-BS system cannot meet such requirements.
Centralized scheduling can be adopted to solve the above problem, wherein a centralized scheduling device is required, and it can either be a dedicated scheduling device or be integrated in a certain base station. Such centralized scheduling device is responsible for determining which mobile terminals are to be served by multi-BS by means of joint pre-coding, determining which mobile terminals are to be served by its serving base station with interference being cancelled by other base stations, and informing each of coordinating base stations of the determined results.
However, the network is required to be flat in some of recent upper layer standards. This is mainly because that there are more levels in the network adopting centralized scheduling and this easily causes high delay in data or signaling transmission, which cannot be accepted by many delay-sensitive services. Additionally, in the centralized scheduling way, RNC is often served as the scheduling device, which causes heavier load to the RNC, whilst the load of the RNC can be reduced by adopting flat, distributed scheduling way.
Although distributed scheduling is preferred by the above upper layer standards, no company has provided any feasible selecting strategy for mobile terminals for distributed scheduling in multi-BS MIMO system.